A method for polymerizing a conjugated diene compound according to anionic polymerization in a hydrocarbon solvent or copolymerizing a conjugated diene compound and a vinyl aromatic compound has been conventionally known.
Although 1,3-butadiene is widely used as the conjugated diene, high-concentration 1,3-butadiene purified by an extractive distillation method using a specific polar solvent such as a GPB method and a BASF method from a C4 fraction obtained in naphtha cracking has been known as a petrochemical product for 1,3-butadiene (for example, see Non-Patent Literature 1).
The above-described 1,3-butadiene is apt to generate a polymer according to radical polymerization referred to as so-called popcorn, and usually contains a polymerization inhibitor such as TBC (t-butylcatechol).
Furthermore, other monomer used for copolymerization with 1,3-butadiene also contains a predetermined polymerization inhibitor.
When 1,3-butadiene and the other monomer are copolymerized, a step of removing the above-described polymerization inhibitor is required. A technique for the step of removing the polymerization inhibitor has been conventionally disclosed (for example, see Non-Patent Literature 2).
FIG. 2 shows a flow chart of a conventional step of removing a polymerization inhibitor and an impurity from 1,3-butadiene to obtain 1,3-butadiene used for anionic polymerization.
The impurity has a profound effect on a polymerization step in the anionic polymerization of 1,3-butadiene. Therefore, as shown in FIG. 2, the polymerization inhibitor is first removed, and a water-washing step is then carried out by using pure water subjected to deaeration treatment through a reduced pressure deaeration tank. A very small amount of impurity is removed by methods such as distillation and adsorption. Furthermore, a dehydrating column step, i.e., dehydrating treatment is then performed to recover 1,3-butadiene. The anionic polymerization is performed by using the 1,3-butadiene and a predetermined solvent.